A number of prior art processes have been proposed for the preparation of dicarboxylic anhydrides, such as maleic anhydride, by the catalytic oxidation of certain hydrocarbons such as the n-butenes, 1,3-butadiene, benzene and n-butane in the presence of air or other oxygen-containing gases at elevated temperatures using phosphorus-vanadium-oxygen catalyst systems which systems generally include other metals or metal compounds as potential yield improving co-catalysts such as molybdenum, copper and uranium oxides or other oxides incorporated into the catalyst system during preparation. Such catalysts are generally not well defined and usually comprise a number of compounds or phases, any one of which may act as the actual oxidation catalyst. These catalyst systems however, as well as other catalysts showing some activity for the production of maleic anhydride have generally proven to be unsatisfactory for commercial application and leave a lot to be desired since the yield of and selectivity to maleic anhydride is usually low. For example, a catalyst system for the preparation of maleic anhydride comprising vanadium pentoxide and oxides of phosphorus and molybdenum as described in Japanese Pat. No. 12,327/1972 to Yokayama et al only gave a yield of 48 percent.
The typical phosphorus-vanadium-oxygen catalyst system is usually prepared by reducing vanadium pentoxide to vanadium (IV) in water or an organic solvent with hydrochloric acid or other suitable reducing agents. A source of phosphorus, usually phosphoric acid, is mixed with the vanadium (IV) solution to produce a catalyst precursor which is heat treated to give the production catalyst. Co-catalysts as above described, for example, are usually incorporated into the catalyst system during the solution stage of preparation.
U.S. Pat. No. 4,062,873 and the numerous patent references noted therein, all of which are deemed to be reiterated herein, describe various vanadium-phosphorus oxide catalyst systems employed in producing maleic anhydride from n-butane and particularly the n-butenes.
U.S. Pat. No. 3,907,835 discloses a gas phase process for the production of maleic anhydride by the catalytic oxidation of an unsaturated hydrocarbon as well as an aldehyde with a gas containing free molecular oxygen under oxidation conditions in the presence of a catalyst of an admixture of vanadium, uranium, phosphorus and oxygen.
One process which has been commercialized is shown in U.S. Pat. No. 3,904,652 which discloses a process for the oxidation of n-butane to form maleic anhydride over a phosphorus-vanadium-oxygen complex catalyst containing one or more metal activators selected from zinc, copper, bismuth or lithium and maintaining a particular concentration of the n-butane in the reaction zone.
U.S. Pat. No. 3,864,280 describes a crystalline phosphorus-vanadium mixed oxide hydrocarbon oxidation catalyst composition consisting primarily of pentavalent phosphorus, vanadium and oxygen, useful for the production of acid anhydrides such as maleic anhydride.
U.S. Pat. No. 3,366,648 discloses a process for the production of maleic anhydride by contacting butene-1, butene-2, butadiene-1,3 or mixtures thereof in the vapor phase at elevated temperatures with oxygen and a vanadium-phosphorus catalyst complex having an atomic ratio of from about 1.0:2.0 atoms of phosphorus per atom of vanadium and as a phosphorus stabilizer an element of Group IA of the Periodic Table.
U.S. Pat. No. 3,293,268 discloses a method for the preparation of maleic anhydride which comprises oxidizing n-butane in the presence of a phosphorus-vanadium-oxygen-containing complex catalyst prepared by reacting phosphoric acid with vanadium oxalate in aqueous solution, by reacting phosphoric acid with ammonium metavanadate in aqueous solution or by reacting a phosphorus compound selected from phosphoric acid and P.sub.2 O.sub.5 with a vanadium compound in an aqueous solution of a hydrogen halide, to give a catalyst having particular gram atoms of phosphorus per gram atom of vanadium.